Replace unicode left single quotation by Ascii single quote

[Bananeweizen]

Use Ascii single quotes on both sides of the user name. Jenkins core uses the unicode character in several places, but this plugin generally doesn't. See line 226 for the finishing quote character.

Testing done

None

### Submitter checklist
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- [x] Ensure that the pull request title represents the desired changelog entry
- [x] Please describe what you did
- [ ] Link to relevant issues in GitHub or Jira
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- [ ] Ensure you have provided tests - that demonstrates feature works or fixes the issue